2,4'-diisocyanatodiphenyl methane and, in particular, 4,4'-diisocyanatodiphenyl methane are important starting materials for the production of polyurethane plastics.
In the condensation of aniline and formaldehyde in the presence of acid catalysts, followed by reaction of the polyamine mixtures formed with phosgene, the isomeric compounds 2,2'-, 2,4'- and 4,4'-diisocyanatodiphenyl methane are simultaneously formed. These isomers have been found to contain varying amounts of impurities, including those which contain organically-bound chlorine in the form of hydrolyzable chlorine. Compounds containing hydrolyzable chlorine can adversely affect the reaction of isocyanates with polyols to form polyurethanes because these chlorine compounds influence the velocity of the reaction and in general, retard it. Accordingly, the hydrolyzable chlorine content of isocyanates is an important parameter when considering the purity thereof. Compounds containing hydrolyzable chlorine include, for example, the reaction products of phosgene with secondary products from the condensation of aniline and formaldehyde, such as p-aminobenzyl aniline, N-methylamino compounds, acridane and acridine (acridine being obtainable from acridane by a redox reaction).
It is known that impurities may be removed from distillable isocyanates by processes which are either based on the addition of compounds capable of converting chlorine-containing impurities into a less volatile form such as (see, e.g. German Pat. No. 1,138,040) or by a technically complex physical separation process (such as crystallization as described in German Offenlegungsschrift No. 1,938,384). As commercial products, diisocyanatodiphenyl methanes, obtained by fractional distillation from a polyisocyanate mixture, are mixtures of isomers. The ratio of the various isomers is governed by the composition of the polyamine mixture used and by the effort involved in the fractionation of the isocyanates.
Because of the low concentration thereof, the multiplicity thereof and to a certain extent, the sensitivity thereof to heat and changes caused by inter-reaction, the chlorine-containing compounds present in the commercially produced diisocyanates are difficult to analyze. The literature describes splitting reactions of N,N-disubstituted carbamic acid chlorides which, depending on the substituents and the presence of catalysts, may take place at temperatures above 150.degree. C., at which temperature, diphenyl methane diisocyanates are generally distilled. Such splitting reactions, in which substantially involatile chlorine-containing compounds are converted into lower boiling compounds [such as conversion of the acid chloride formed from phosgene and p-aminobenzyl aniline into p-isocyanatobenzyl chloride and phenyl isocyanate (J. Org. Chem. 39 (1974), 2897-2899)], also alter the chlorine function. Thus, a readily hydrolyzable chlorine in a carbamic acid chloride becomes a more firmly bound chlorine in a benzyl chloride function.
In conventional distillation-based purification processes (German Auslegeschrift No. 1,923,214), separation of the solvents used for the amine/phosgene reaction is generally followed by purification in the following stages: Removal of diisocyanate from a polyisocyanate mixture by distillation, separation of the isomeric diphenyl methane diisocyanates by fractional distillation in a column and distilling off diisocyanate of reduced 2,4'-diisocyanatodiphenyl methane content from polymerization products which are reformed during distillation by the action of heat on isocyanates. The chlorine-containing impurities are only unsatisfactorily separated off by this process, probably because reactions of the type described above continuously take place during the thermal stressing of the distillation process.